RTOA based systems typically attempt to locate a certain device based on the time taken for signals to propagate between that certain device and base stations in communication therewith whose locations are known. This is made possible by the fact that signal travel time is directly related to the distance traveled by the signal.
Some systems, such as GPS and LORAN-C, are "outbound," meaning that the signal is emitted by the base stations and received by the portable or mobile unit which calculates its location. Other systems are "inbound," meaning that the signal is transmitted by the mobile unit and picked up by the receivers, which typically report the arrival time to a central unit where the location calculation is performed. With a strictly inbound or outbound signal, the travel time is not known; the only known value is the difference in travel times between a given device (e.g., a mobile radio unit) and a plurality of base stations.
RTOA systems typically employ either hyperbolic triangulation or trilateration. In trilateration, such as employed in GPS, the position of an unknown point is determined by measuring the lengths of the sides of a triangle between the unknown point (e.g., a mobile unit) and two or more known points (i.e., satellites). For triangulation type position determinations, a position is determined by taking angular bearings from two points a known distance apart and computing the unknown point's position from the resultant triangle. In RTOA systems, because a specific distance difference between a certain target (receiver) device and two fixed points in a plane (i.e., satellites (GPS) or ground based stations (LORAN-C)) define a hyperbola, relative position information by and of the target can be readily determined. As is well known and understood, the intersection of two or more hyperbolae defines the geometric positional location of the target device. Thus, by making available additional satellites (or ground based stations) a more accurate position can be calculated than with fewer satellites. Generally, it is possible to determine three-dimensional position in GPS using a maximum of only four satellites.
Referring to a GPS system as an exemplary RTOA system, satellites are selectively employed which transmit a coded radio signal that is unique to each satellite. Mobile units on the ground passively receive each visible satellite's radio signal and measure the time it takes for the signal to travel from the source (satellite) to the target (the receiver itself). In its most simplistic form, determining distance is a simple matter of computing distance by multiplying the time in transit of the coded radio signal by the velocity of transit. In an RF environment, since radio waves travel at the speed of light, which is essentially fixed at 300,000 km/sec., the velocity is known. The only thing needed by the receiver to calculate distance from any given satellite is a measurement of the time it took for a coded radio signal to reach it from the satellite.
The critical feature, therefore, in RTOA systems is that appropriate clocking means be provided such that the arrival time of transmitted signals can be recorded/determined with extreme precision. To acquire an accurate position it is crucial that very precise time measurements can be made. In GPS, it turns out that it only takes about 1/15 of a second for a satellite signal from orbit to reach a receiver on the ground. Doing the math, an error of one nanosecond in an RF system translates into a location error of approximately a foot. The arrival of signals at different receivers for inbound systems (or the transmission of a signal in outbound systems) must be known relatively within the system resolution, typically 10-300 ns for an RF system. Any error attributed to synchronization translates into additional location error.
Most prior art systems, such as GPS and LORAN-C, circumvent this problem with an outbound architecture. LORAN-C systems synchronize constantly from a master clock, and GPS systems are provided with satellite-carried atomic clock timebases. Atomic clocks are the choice time source as they accurate to within billionths of a second per month. However, besides weighing hundreds of kilograms each, each clock costs in the hundreds of thousands of dollars. While cost and weight may be acceptable for outbound systems where a limited number of such clocks may be required (i.e., in only each of a limited number of available satellite (GPS) or ground based stations (LORAN-C) transmitting coded radio signals to for example ground-based receivers), it is not practical to employ expensive clock sources, such as atomic clocks in lower-powered systems where the number of receivers (base stations) is much higher.
A ground-based inbound system 100 employing conventional time-of-arrival position location measurements, is depicted in FIG. 1. In the exemplary system 100, there is included a plurality of base station receivers 121, 122, 123, and 124 coupled to receive a signal broadcast from a transmitter 130, which signals travels over direct paths 141, 142, 143, and 144 to each of the base station receivers 121-124, respectively. The transmitter 130 may be a cellular phone, pager or the like portable device, or could be an agricultural crop row monitor, or even a non-terrestrial object, so long as provided with means to transmit a signal detectable by the base station receivers 121-124. It should be likewise appreciated that the base stations could be ground-based, as shown in the figure, or nonground based (e.g., satellites) type devices, or a combination of both ground and non-ground based devices, and are capable of both signal transmission as well as signal reception. For ease of comprehension, the transmitter 130, while it may typically be employed as a receiver device (e.g., a cellular phone), in the context of an inbound position location RTOA system such as shown in FIG. 1, it is given the label of "transmitter" since for the purpose of having its position identified by the system it must necessarily function as a transmitting device. Likewise, the base station receivers 121-124, while typically functioning as transmitter devices, sending voice and/or data signals to mobile units such as transmitter 130, are characterized as receivers, since in the context of an inbound system, function as receivers of the signal transmitted by the transmitter 130 over the associated link 141-144.
Referring back to FIG. 1, each base station receiver 121-124 reads its own independently-running clock 146-149 when the signal is received over direct paths 141-144, respectively, timestamps the time the signal was received and reports the time of arrival timestamp of the signal over a separate link (via, for example, a corresponding wireline modem link 151, 152, 153, and 154) to a navigation console 181. The navigation console 181 computes the differences in the arrival times and uses them to calculate a location of the transmitter 130. In the illustrative embodiment of FIG. 1, it is assumed that each base station clock 146-149 is an atomic clock or the like high precision timebase source so as to independently report very precise, high-accuracy time-of-arrival signals to the console 181.
The use of plural high accuracy clocks inherently significantly impacts the cost of system implementation in the typical inbound system as depicted in FIG. 1.
Certain inbound systems address the problem of atomic clock deployment at each and every critical receiver, by substituting where practically possible, oven-controlled oscillators, and by frequently synchronizing transmissions from a reference device, thus tracking offset and drift from a central location, or a combination of both. However, oven-controlled oscillators of the caliber required for very precise time broadcast also have a severe cost impact, and are subject to drastic environmental restrictions. For example, it is not unusual that in order to maintain proper oscillator operation (accurate timing), many such oven-controller oscillators require that they be placed in a very expensive climate-controlled environment.
Lastly, even where very precise time sources are provided in an inbound type system, unaccounted clock drift that may occur between synchronization transmissions, adding to the overall system error, needs to be accounted for. Accordingly, synchronization transmissions consume bandwidth and power without adding any value to the system.
Thus, although the timebase requirements raise cost, reliability, and often times environmental issues, the approach of refining the existing solution rather than seeking a new solution has been the preferred choice. The timebase issue is generally seen as "solved" even if the solution is less than perfect, by a solution that requires the system developer to first specify the desired accuracy and otherwise accept the cost penalty for achieving that accuracy.
It would therefore be a great advancement in the art to be able to report useful time of arrival signals of a signal broadcast from a transmitter to plural remote receivers for use by a central station without having to employ independently running high-precision clocks at each receiver.